The oath of office was once religion-neutral

The phrase “so help me God” is a modification to the neutral oath of office outlined in the U.S. Constitution:

Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:–’I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.’

— U.S. Constitution, Article II, Section 1